I've got a plenty of those messages in my logs. The source of some of them I recognised, while others I didn't.
I think that all these messages appear when I play the game with my own language.
First of all, it floods logs when the geoscape map is open in late game. I tried to reproduce it starting new game and I didn't succeded. I will upload savegame that has these messages.

Second, these messages appear when I hover over items in equipment menu and these tooltips are trimmed because length of tooltip is too small. I already explained it in IRC so I just copy-paste from there and format it better.

At first it's adding the ammo description, then for which weapons this ammo is suitable. So we have only 25 chars left - its (128 - $(length of ammo description)). And there it gets trimmed because tooltip length is 128 and it's not enough to fit in size. There goes first Q_vsnprintf warning. Then it tries to add debug info ( x pos / y pos ) and it doesn't fit the size too. So there goes the second warning.